Magnetic recording media such as magnetic tapes are commonly prepared by applying to a support a magnetic coating composition comprising a magnetic powder, a binder resin and so forth, followed by drying. In conventional magnetic recording media, only one magnetic layer is used, and hence a wide frequency band ranging from a low band to a high band must be covered using one kind of magnetic powder. In particular, in the recent tendency toward higher recording density, it is required to provide a magnetic recording medium improved in recording characteristics at a higher band and yet having a low noise, and hence a magnetic powder with a high Hc and a high BET value is used.
However, because of the magnetic recording medium comprised of one kind of magnetic powder (or magnetic layer), too much account of high-band characteristics is made to avoid using a magnetic powder of a high Hc and high BET value, resulting in insufficient characteristics at low bands.
On one hand, in magnetic recording media used for video tapes, proposals have been made on a medium having a plurality of magnetic layers so that magnetic recording capacity can be enhanced, or magnetic recording characteristics at both the high-frequency bands and low-frequency bands of a medium can be improved and balanced (see Japanese Patent Publications Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) No. 98803/1973 and No. 172142/1984, Japanese Patent Examined Publication No. 2218/1957, Japanese Patent O.P.I. Publication No. 64901/1976, Japanese Patent Examined Publication No. 12937/1981, and Japanese Patent O.P.I. Publications No. 56228/1983 and No. 146221/1988, etc.)
According to these known techniques, a magnetic recording medium comprises magnetic layers comprised of an upper layer in which a magnetic powder with relatively fine particles is used and a lower layer in which a magnetic powder with larger particles than that, and are so designed that the upper layer serves video outputs and the lower layer serves chroma-audio outputs.
On the other hand, Japanese Patent O.P.I. Publication No. 61230/1986 discloses a magnetic recording medium comprising a magnetic layer containing a magnetic powder comprising Co-containing .gamma.-Fe.sub.2 O.sub.3 which contains a divalent iron component (Fe.sup.2+) in an amount of from 3.5 to 10% by weight in terms of FeO and from 0.04 to 0.123 in terms of Fe.sup.2+ /Fe.sup.3+. Japanese Patent O.P.I. Publications No. 19524/1989, No. 86321/1989, No. 13225/1989 and No. 300425/1988 also disclose that FeO.sub.x (where 1.33.ltoreq.x.ltoreq.1.50) is used as a magnetic powder.
As a magnetic powder, a Co-containing iron oxide comprising Fe.sup.2+ of as much as Fe.sup.2+ /Fe.sup.3+ =0.13 to 0.45, compared with Co-containing .gamma.-Fe.sub.2 O.sub.3 comprising Fe.sup.2+ in a proportion of Fe.sup.2+ /Fe.sup.3+ =0.001 to 0.1, is called a magnetite type (Fe.sub.3 O.sub.4), and is comprised of FeO and Fe.sub.2 O.sub.3. Such a magnetite type Co-containing iron oxide commonly has superior properties such that it is black in color and superior in light-shading properties, and hence the amount of carbon black to be added in a medium can be reduced, can readily make exchange of electrons between Fe.sup.2+ and Fe.sup.3+ to lower surface specific resistance of a magnetic layer, and yet shows a coercive force (Hc) with a value which is advantageous for chroma outputs.
However, employment of the Co-containing iron oxide of a magnetite type results in an increase in the proportion of FeO, and hence the properties of the surfaces of magnetic powder particles turn electron-donative to cause changes of adsorption behavior at the time of dispersion. This causes deterioration of dispersibility and tends to result in a lowering of electromagnetic conversion characteristics. This may also cause an increase in the adsorption of a fatty acid to a magnetic material, so that the fatty acid that may exude to the surface of a medium can be controlled with difficulty. This tends to cause faulty travel, or contamination of a calender roll at the time of manufacture.